1. Field
This invention relates generally to amplifiers, and more specifically to an output structure of an amplifier.
2. Related Art
The input of some circuits may appear very capacitive to an output structure that is driving the circuit. The input impedance of such circuits decreases as the frequency of a signal driving such circuit increases. As the frequency of the signal driving such circuit increases, a current consumed by some known output structures increases. Some known output structures disadvantageously consume a large amount of current regardless of the driving frequency.
Many known output structures include a differential pair gain stage or another type of class-A gain stage. The greatest amount of current that a differential pair of transistors (hereinafter “differential pair”) can deliver to a circuit being driven by the output structure is the amount of current used to bias the transistors of the differential pair. Therefore, in some known output structures, when the circuit being driven has a high input capacitance, a tail current of the differential pair must be large at all times, thereby disadvantageously constantly consuming much current.
With a conventional differential pair, a maximum transient current through either input transistor is equal to a magnitude of the fixed tail current. The fixed tail current imposes a slew rate limitation when the differential pair is subjected to large-signal transient input signals. Known output structures of low-power CMOS amplifiers have large-signal response time limited by slew rate. Slew rate of an amplifier is a measure of how rapidly an amplifier can charge/discharge a capacitance that is coupled to an output terminal of the amplifier in response to a rapid large-signal increase/decrease of an input signal of the amplifier. Slew rate of an amplifier is approximately equal to a fixed bias current divided by a capacitance of an input impedance of a circuit being driven by the amplifier. A large signal frequency response of an amplifier depends on the slew rate.
A differential voltage at an input of a class AB amplifier is converted into an output current. A class AB amplifier needs to provide a larger current at its output when reacting to large signal disturbances than when reacting to small disturbances or during steady state.